In a study of 301 consecutive fetal heart rate (FHR) recordings of greater than or equal to 6 h duration, consecutive episodes of low and high FHR variability consistent with cycles of quiet and active fetal behaviour were found in 38% of spontaneous and 68% of induced labours at term (P less than 0.0001). Quiet periods (episodes of low FHR variability) had durations of 12-93 min (mean 25, SD 11 min) and in 48% of cardiotocographs (CTGs) these episodes had a long-term variability of less than 5 beats/min. The implications of these findings are discussed in relation to the use of FHR recording for intrapartum surveillance.